In seeking maximum possible SPL from acoustic transducers such as heavy duty low frequency loudspeakers, it has been found empirically in tests and studies of examples of the best of known art, that there appears to be a "piston band" wall or barrier that has heretofore limited the obtainable SPL to just under 120 dB/1 m (sound pressure level of 120 dB referred to 20 micropascals, measured at a distance of 1 meter from the loudspeaker) regardless of differences in design approaches and variations in efficiency, magnetic flux, voice coil form factor, size, etc.
Temperature plays a key role in this limitation: as the SPL is increased, the I.sup.2 R power loss dissipated in the voice coil increases. This increase is accelerated by the positive TCR (temperature coefficient of resistance) of the metal voice coil wire. To the extent that the resultant heat is not removed immediately, the temperature of the voice coil rises. If sufficient heat sinking is provided the temperature will stabilize at a point of thermal equilibrium, otherwise a thermal runaway condition will result in the temperature rising continuously to an ultimate level of destruction.
The maximum available SPL is limited to that producing a maximum working temperature level of sustainable equilibrium that approaches, with an acceptable margin of safety, a potentially destructive ultimate temperature limit determined by such factors as thermal properties of adhesives, bobbins and other voice coil materials. Differential expansions, distortions, can distort the voice coil dimensionally to the point of destructive interference with surrounding magnet poles, depending on pole gap clearances, and repeated expansion/contraction from temperature cycling can cause deterioration and shortened useful life of the loudspeaker.
In the case of constant voltage drive, the increasing coil resistance reduces the current, the power efficiency, and the acoustic power output, and accordingly limits the maximum available SPL.
In the case of constant current drive, the I.sup.2 R power dissipation increases regeneratively because as I remains constant R increases, further increasing dissipation and temperature. This potentially destructive runaway condition is at best difficult and at worst impossible to control with conventional heat removal systems, given the unfavorable heat sinking characteristics of the moving voice coil structure as contrasted with fixed coils such as those in transformers where heat-sinking of the windings can be enhanced, for example by encapsulation in heat-conductive materials.
The wire most commonly used in voice coils is made from copper or aluminum, both of which have a positive TCR of 0.0041 (20.degree.-100.degree. C.) in pure form. Conservative design practice addresses the worst case of continuous maximum power over a prolonged period of time, along with a high ambient temperature, even though the long-term average loading factor from typical voice and music operation may be relatively low.
Designers have adopted copper and aluminum (and occasionally silver) for voice coil windings almost exclusively on the basis of low resistivity at room temperature (20.degree. C.), and have simply accepted the TCR resistance rise. The potential of utilizing wire material with lower TCR and suitable density, despite higher initial resistivity, has not been recognized heretofore.